Colistin antimicrobial susceptibility testing—can the slow and challenging be replaced by the rapid and convenient?

Abstract

Abstract Colistin antimicrobial susceptibility testing (AST) has been acknowledged as challenging, and recently EUCAST has posted warnings regarding the gradient tests, which have been used for testing by many clinical laboratories. Current EUCAST recommendations state that only broth microdilution (BMD) should be used for AST of colistin. In a recently published paper three methods for colistin AST were compared against a collection of Enterobacteriaceae clinical isolates (83 colistin-resistant and 40 colistin-susceptible). In addition to BMD used as a reference, the authors used the Phoenix automated AST method, and the Rapid Polymyxin NP test. The latter is a colorimetric test based on formation of acid metabolites consecutive to the glucose metabolism as a sign of growth in the presence of colistin (colistin resistance). The Phoenix system failed to detect ten colistin-resistant isolates (one Escherichia coli , one Klebsiella pneumoniae , seven Enterobacter spp., and one Salmonella enterica ), whereas The Rapid Polymyxin NP test failed to detect the same single E. coli isolate. The EUCAST Development Laboratory will investigate the performance of the Rapid Polymyxin NP test in a separate investigation versus a strain collection with low-level resistance to colistin. Based on the results in this investigation there will be a decision on whether the Rapid Polymyxin NP test can be recommended by EUCAST as one possible method for colistin AST, and whether there are some caveats. Meanwhile, clinical laboratories, particularly larger university laboratories with complex patients, should acknowledge the increasing need for reference AST, and preferably implement BMD.